Polytetrafluoroethylene (PTFE) is an ideal material for use in industrial, medical, automotive and consumer electronics. Specifically, PTFE, fluorinated ethylene propylene (FEP) and other PTFE compounds have outstanding physical properties; such as chemical inertness and resistance to chemical corrosion, even when exposed to a variety of chemicals, such as a strong acid, an alkali and oxidants. Their physical properties provide superior electrical insulation and thermal stability, which is not affected by wide ranges in temperature and frequency. Their resistance to absorption of moisture makes them a perfect material for consideration in micro optical, retro-reflector or diffuser type devices used in handheld displays, flat panel displays as well as a variety of automotive, industrial, home lighting, and other applications where their properties are well suited are medical diagnostic devices, biochip, fluidic channels and micro-channel plates for electrophoresis.
Isostatic molding is the only process which is used for standard forming of PTFE and PTFE compound materials for macro-structures or to form large blocks which are subsequently machined, using traditional machine tools, into other macro products such as bearings, housings, etc. Isostatic molding of PTFE, FEP and PTFE compounds is typically achieved by placing a powdered compound into a mold form with one side of the mold applying a compression force which compresses the powder to closely conform to the shape and profile of the mold.
Standard isostatic or compression molding, bonding or forming processes rely on the assembly of parts or joints which are then subjected to isostatic or directional pressure, while simultaneously elevating the parts to its fusion temperature and maintaining this pressure through to the solidification phase forming an integral monolithic structure of pure PTFE, FEP or compounds of PTFE.
There are several methods that are used to create an isostatic or compression mold which can achieve the pressure required to form a desired component. Certain techniques utilize the differential thermal expansion between the PTFE and the material forming the mold, such as aluminum.
This allows the pressure to be achieved without having to use active pressure via a hydraulic press. However, this type of technique has limitations when the features or details being molded fall below standard surfaces finishes and where the aspect ratio of the micro or nano structures extends above the 1:1 aspect ratio. This type of mold is then either placed into a furnace or dunked into a molten bath of salt to raise the temperature quickly. Once a sufficient pressure and temperature for fusion is achieved, the mold is then quenched into water and allowed to cool. This quenching allows the mold to be cooled quickly and also sufficiently shocks the part being molded and this allows the part to be easily released from the mold.
The drawback of standard isostatic or compression molding, other than the limitations due to aspect ratio, is that the material density, any trapped air or other contaminant does not allow this standard isostatic molding to form sufficiently uniform features below two hundred (200 μm) microns. One primary reason is trapped air and other contaminants, such as trapped moisture, have a tendency to create voids, inclusions and/or change the density of the material. The inventors have discovered that by applying a vacuum to the mold during the molding process, any trapped air and/or moisture can be removed and this, in turn, also assists with self compression of the powder or palletized PTFE or PTFE compound self compresses, and eliminates potential voids and thereby allowing the mold anvil to compress the material to even higher densities than previously possible. This also allows tighter control of surface finish and dimensional control of the features being produced.